1. Field of the Invention
The present invention relates to a fluid coupling with a lock-up clutch and especially relates to a fluid coupling with a lock-up clutch used in an automobile as a torque converter.
2. Description of the Related Art
A fluid coupling with a lock-up clutch is commonly used in an automobile as a torque converter.
In a known fluid coupling with a lock-up clutch, a drive disc assembly, which is connected to a front cover through damping mechanism, is disposed close to the front cover and a driven plate assembly which is connected to a hub at an inner end is disposed between the drive disc assembly and a turbine shell. The front cover of the fluid coupling is fixed to an input shaft and the hub is fixed to an output shaft (Japanese Unexamined Patent Publication "kOKAI" 5-187518). In this type of fluid coupling with a lock-up clutch, a front drive disc and a rear drive disc, which are the components of the drive disc assembly, are connected to each other only at their midpoints. Therefore, the drive disc assembly does not have high rigidity and is easy to deform.
As a result, the drive disc assembly and the driven plate assembly are not engaged to each other at constant position, and the torque transmitting capacity in a lock-up condition is low. Therefore, it is necessary to increase the thickness of the drive discs to obtain higher rigidity. This causes an increase in the axial length of the automatic transmission, which results in poor mountability of the automatic transmission in a body of automobile, and increase in weight which results in poor fuel consumption.
In other fluid couplings with a lock-up clutches, the driven plate is disposed directly on the outer surface of turbine shell and in a radially outer area (U.S Pat. No. 5,195,621). In this type of fluid coupling with a lock-up clutch, turbine blades are fixed to the inner surface of a turbine shell throughout the whole radial length of the turbine shell by inserting one end of the turbine blade into slits which are formed on the turbine shell and then brazing the blades. Therefore, if the brazing is broken by repeated deformation of the turbine shell, the oil filled on the inner side of the turbine shell will leak to the space between the outer surface of the turbine shell and the drive disc assembly. As a result, it would be difficult to obtain desirably low oil pressure in the space between the outer surface of the turbine shell and the drive disc assembly, so that the torque transmitting capacity of the fluid coupling at a lock-up will be low.